This invention relates to a method and apparatus for detecting when the value of a measured variable has reached a plateau or, in other words, has reached a value which remains substantially constant over a period of time. The apparatus and method deals specifically with the detecting of the plateau when the measured variable is represented by a periodically produced string of pulses wherein the pulse count of the string is representative of the value of the measured variable. An example of the type of application in which a plateau detecting circuit can be used is the temperature measurement of steel in a refining furnace as, for example, by the use of the well known expendable thermocouples which are immersed in the steel bath for the purpose of making a single temperature measurement of the bath.
Prior art devices known to the inventor have usually included electromechanical elements and have not been capable of detecting the plateau when the variable is represented by a digital signal comprised of a string of pulses whose pulse count represents the value of the variable.